The generators and loads in an electrical power network are often located far apart from one another and are connected through distribution units and a transmission network. The generators operate in parallel to continuously supply the loads. The interconnections between multiple generators, networks and loads have multiplied the complexity to solve the engineering problems in a power system. The generators generate electrical energy from other forms of energy such as coal, hydro, wind or fossil fuel, which are interconnected through networks of transmission lines called an electric grid. The transmission network transmits the bulk amount of generated energy from one location to another location at higher voltage levels. The distribution units distribute the energy to the consumers at lower voltage levels.
The electrical energy balance between generation and consumption is important for power system operation. However, the consumption or the demand of electricity is usually random and the power system has no control over it. To accommodate these changes, the power system is equipped with power generation and flow control devices throughout the transmission network and the generators (such as an excitation system, governor, regulating transformers, etc.). These devices help to achieve the required operation of the power system by maintaining voltage, frequency and other system variables within the values defined by power generation standards.
Disturbances such as faults, load changes, generator input changes, line trip-outs and the like in a power system cause system variables to deviate from the normal values. The deviations due to small disturbances can be handled by control devices which bring them back to a normal condition. However, the control devices cannot handle changes due to large disturbances which lead the power system to an abnormal condition which may in turn lead to an unstable situation. For example, an imbalance between the input mechanical power and the output electrical power due to the disturbance causes generators in one region to run faster than the generators in another region. This results in angular separation between the rotors of these generators in these two regions, which keeps on increasing if the power system cannot absorb the kinetic energy corresponding to the rotor speed differences. If the angular separation exceeds 180 degrees, the generators in the two regions lose synchronization. This is called an Out-Of-Step, Pole-Slipping or transient instability condition. To deal with such a condition, out-of-step tripping needs to be initiated for selected breakers in the power system located on transmission lines or at the generator terminals. Out-of-step tripping is now receiving more attention because of the very large generators, long transmission lines that are connected to extra high voltage and ultra-high voltage circuits. However, the lower inertias and the higher reactances of the generators that are used today have reduced the stability limit of the power systems. Also more and more power is being transmitted on the lines, which has reduced the stability (or security margins of the system). Therefore, if the out-of-step condition is not identified in time, it initiates undesired tripping of transmission line relays, cascade outage of generators, and a wide area blackout, which has a severe technical, economic and social impact.
A power system is a complex dynamic system with multiple generators, loads, motors and other fast-acting power electronic units which together form a highly nonlinear system, where each unit has different characteristics and responses. All of the dynamic units have to be steady state stable in order to ensure the steady operation of the whole power system. The power system is stable if it can absorb any excess energy developed due to disturbances, and return to the previous stable state or to a new stable operating state. However, when a disturbance happens, the power system might suffer from different forms of stability issues, which are classified into three major areas: rotor angle stability, frequency stability and voltage stability. Further classification of power system stability is based on the strength of the disturbance and the time duration to be considered for the stability studies.
The power system disturbances are an abnormal situation that causes the power system state to move from its steady state equilibrium. Small disturbances such as continuous load changes result in small shifts in power system state from which the power system can recover. However, the large disturbances, such as faults, major line tripping, loss of generation or huge load changes, result in a large shift in the states of the power system and result in high oscillations in voltage and current throughout the power system. For example, in the case of a large disturbance, the power output from a synchronous machine starts fluctuating, which causes the rotor of the synchronous machine to accelerate and decelerate with respect to the stator circuit. As a result, the synchronous machine starts oscillating with other synchronous machines in the power system. If the power system has sufficient synchronizing and damping energy, the oscillations dampen out and settle to an equilibrium state in a finite time. This implies stable operation of the power system. However, in the case of the power system not being able to dampen the oscillations, an unstable situation arises from which the power system cannot return to a steady state and the generator rotor angles keep separating from each other resulting in the out-of-step condition. This is also referred to as loss of synchronism or rotor angle instability. If the out-of-step condition is not detected quickly, it can have a cascading effect, such as unnecessary tripping of other major lines, generator tripping and so on. The situation, therefore, demands a fast out-of-step protection strategy to be designed and implemented in the power system.
Power system protection consists of a system of entities that protect the power system components, such as generators, transformers, transmission lines, and the devices at the consumer level from the high system currents and voltages during or after a power system disturbance. The main function of the protection system is to ensure the prompt isolation of the power system components that might cause damage or otherwise interfere in the effective operation of the rest of the power system. The entities that provide protection (i.e. protective devices) mainly include instrument transformers (e.g. Current Transformers (CTs) and Potential Transformers (PTs)), protective relays, breakers and communication devices. Instrument transformers act as sensors which sense current and voltage and feed them to the protective relays. A protective relay acts as the brain of the protection system since it detects dangerous and intolerable situations in the power system via sensors and makes decisions using operating principles or past experiences to perform corrective action as soon as possible. The protective relay usually protects only a certain portion of the power system. Accordingly, the power system is divided into various overlapping sections called zones of protection which are each protected by a protective relay. A zone of protection is normally defined for a generator, transformer, substation, transmission line, distribution line or a motor. An edge of a zone of protection is defined by the CTs through which the associated protective relay sees (i.e. monitors) the power system inside the zone of protection. Communication devices can be used to establish continuous communication between two or more protective relays to ensure a coordinated operation of the whole protection system.